Answer
See explanation below.
Work Step by Step
A cusp can be noticed on the graph at which the slope of a function $f(x)$ approaches $\infty$ from one side and $-\infty$ from the other side.
Let us consider an example:
$f(x)=x^{2/3}$ at $x=0$
$f'(x)=\dfrac{2}{3} \dfrac{1}{x^{1/3}}$
Thus, $\lim\limits_{x \to 0^{+}}f'(x) =+\infty$ and $\lim\limits_{x \to 0^{-}}f'(x) =-\infty$
